Generally, a vehicle is provided with a fuel pump submerged in fuel in a fuel tank so that the fuel in the fuel tank may be pressure-fed to be fed to an engine side, and a fuel pump for the vehicle is integrally coupled to a DC motor to drive an impeller provided in the fuel pump.
As illustrated in FIG. 1, a stator 20 formed of a permanent magnet is fixedly coupled to an inner circumferential surface of the housing 10 and has a first permanent magnet 21 and a second permanent magnet 22 that are disposed to face each other, in which an N pole of the first permanent magnet 21 is disposed inwardly in a radial direction and an S pole of the second permanent magnet 22 is disposed inwardly in a radial direction. At this point, the stator 20 and the rotor 30 are spaced apart from each other, and a first gap g1 between the first permanent magnet 21 and the rotor 30 is equal to a second gap g2 between the second permanent magnet 22 and the rotor 30.
By the way, the DC motor for a fuel pump for a vehicle has different magnitudes of magneto-resistance (enough magnitude to obstruct a flow of magnetic flux) depending on rotated positions of the rotor when the rotor is rotated, and the magneto-resistance causes a pulsation of a motor torque. The pulsating phenomenon of the torque is called a cogging torque, and the motor has a vibration source for vibration and noise due to the pulsation of the torque. Further, the first gap g1 between the first permanent magnet 21 and the rotor 30 is equal to the second gap g2 between the second permanent magnet 22 and the rotor 30, and therefore, a torque ripple which is a fluctuation width of the cogging torque due to the rotation of the rotor becomes large.
Consequently, since vibration and noise occurrence in the fuel pump driven by the motor may be caused, there is a need to improve the noise and vibration characteristics of the motor by reducing the torque ripple of the motor.